Typically, modern electrical products incorporate printed circuit assemblies (PCAs), such as printed circuit boards (PCBs). The range of products is immense, including cell phones, laptops, televisions, MP3 players, game consoles, personal data assistants and aircraft components, to name just a few.
The printed circuits within these products interconnect a variety of circuit components, such as diodes, transistors, resistors, integrated circuits and the like. Fabricated as individual components, each generally has one or more balls, legs or pins (commonly referred to as interconnects). The individual components are brought into useful harmony by a circuit board that provides electrical traces to and from different components as well as areas that facilitate the permanent mounting of components upon the board.
Due to fabrication complexity of many products, the PCAs are assembled in stages. A given PCA and at least some of the components thereon may therefore be subjected to repeated processing steps. As such, the components frequently require monitoring and testing during the fabrication process to ensure that the ultimate device is functional. If an uncorrectable defect is detected early, additional fabrication costs may be saved by halting further assembly of the defective product.
Electrical test probes are used to provide electrical connections between PCA components and testing instruments. An electrical test probe generally consists of an electrically-conductive probing tip joined to an electrically-conductive shaft that is, in turn, connected to a test fixture, which attempts to align the probe to a specific component.
Generally speaking, the components are attached to the PCA by solder. Economic and environmental factors have effected a change in the solder process from lead-based solder to lead-free solder. The use of lead-free solder imposes additional fabrication issues upon the assembly and testing process. For through-hole-technology (THT) components, the process and costs of wave soldering can be eliminated by assembling these to the board using through-hole reflow (THR). THR is a way to mount THT components simultaneously with the surface-mount-technology (SMT) components. In other situations, a through-hole, or via, may connect a designated test pad on one side of the PCA, with a trace or-mounting pad for a component on the other side of the PCA or trace internal to the PCA. To provide good test pad surfacing, solder reflow is generally the preferred method of applying solder to the test pad area.
In addition, as the industry further miniaturizes devices, packing more components within the same area, it becomes more and more difficult to provide accessible test points of suitable size to ensure quality testing is achieved. Moreover, while there is a desire to maximize components in minimum space, this desire is somewhat offset by the need to insure that test points are provided with sufficient access for testing.
Typically, the solder is applied in a paste form with the use of a stencil to the circuit-board. In some instances, components are then pressed into the solder paste, and/or into holes in the board along with solder paste. In other instances, the solder paste is simply applied to provide a-good test pad surface for test probe contact. The board is then heated to the solder melt temperature to reflow the solder such that it wets a pad surface and/or flows about the pins of a component to be joined to the board.
In addition to the solder metal, the solder paste also contains a combination of chemicals, called flux, which help keep the solder in a paste form, act as adhesive so the paste sticks to the pads and pins, thereby holding the components on the board before being reflowed, and clean the metal of pads and pins in order to achieve a good solder joint. The reflowing process releases the flux components of the paste and leaves flux residue on the board and solder joints. The flux residue is a combination of non-conductive materials.
Holes in the board are frequently used to mount components and/or provide board interconnections. When the reflowed solder flows into these holes, it may partially or completely fill them. Flux material also will flow into the hole and gather on top of the reflowed solder. The flux material may lie below the pad of the hole, be flush with it, or flood over it.
In many instances, the hole, or via, as it is more commonly known, is intended to permit electrical connectivity from one side of the board to the other for component testing purposes. Specifically, as the components attached to one side may effectively block or reduce the space available for testing pads, it is frequently common to have the testing pads provided on the opposite side of the board. To provide a quality surface for testing, solder paste and reflowing operations are performed on the test pads of such vias.
When the hole and/or its surrounding pad are the target of a test probe, the flux residue may prevent a reliable and-repeatable electrical connection between the pad and the target when urged with each other. Also, a certain amount of force is generally used when the test probe tip is urged into the solder. If too much force is applied, this may break solder joints, components or the board itself. If too little force is applied, the probe may not make sufficient contact with the solder and a valid component may be judged to be defective. Thus, a low force that repeatedly makes good electrical contact between a test probe and its target is desirable.
As conventional through holes have a consistent diameter, during the reflow process, solder and flux will flow into the hole and the flux having been separated from the solder, will pool on top of the solder. The solder within the hole will generally form a concave meniscuses, which is in turn filled by the flux residue. The flux is, of course, non-conductive. The flux typically will pool and overflow the hole so as to cover and effectively seal the test pad contacts as well.
As most conventional test probe tips are generally in the shape of a cone or other shape that narrows to a point, the pooled flux may well frustrate the ability to make electrical contact between the probe tip and the test pad. As such, the testing may fail despite the node actually being properly functional.
Probe tips in the shapes of cups, crowns and radial stars with three or more tips for alignment over mounded solder elements also exist. However, as the number of contact points increases, so too does the surface area of contact. More specifically, as the points of contact increase, the concentration of applied force transferred to each point decreases.
Thus, the multiplicity of points of contact from start tips, crown tips or the like may further frustrate the attempt to achieve a proper electrical contact between the probe tip and the solder if blocking flux residue is present. Single flat blade probe tips are likewise also frustrated by the presence of flux, as they provide a large surface area for contact and, thus, result in lower contact pressure per unit of contact area.
In short, flux residue presents a serious issue for test probes in that the flux may foul the probe tip and so retard later testing ability, and the flux may prevent electrical contact between the probe tip and the test pad, thus resulting in a potentially erroneously failed test.
Consequently, the necessary electrical contact between the probe and the solder is not achieved in all situations and the testing system may wrongly evaluate a healthy board and/or component as defective, due simply to the contact failure. Also, bad contact may lead to incorrectly passing a bad board. Such incorrect evaluations are costly, either due to costly troubleshooting involved, good product becoming scrapped, or profitability being impacted by bad product becoming deployed and, in turn, necessitating costly customer support under warranty.
Hence, there is a need for a through-hole via that overcomes one or more of the drawbacks identified above.